The present invention relates, in general, to a well logging method using magnetic field sensors to locate subterranean well casings and drill strings in a target well from a nearby borehole, and more particularly to a method for determining the distance and the direction to a first borehole containing material with remanent magnetization from measurements of the static magnetic field in an adjacent second borehole.
The need for locating target wells containing casings or drill strings arises in various contexts, including the need to drill a borehole to intersect or to reenter the target well in question. For example, in the development of gas storage fields, reentry of old wells which cannot be reentered directly from the surface because of corroded casing or because sections of casing have been removed, is an important requirement, particularly for proper abandonment of such wells. Furthermore, the corrosion of poorly abandoned wells in general, and subsequent leakage of these wells into aquifers and to the surface, present problems of increasing environmental significance. Additionally, in the drilling of oil wells, blowouts occur from time to time, and this may require the drilling of a relief well to intersect the blowout target well in order to shut off dangerous high rates of flow, as in cases when surface control is not possible.
Several methods have been developed to measure and to analyze the static magnetic fields associated with ferromagnetic well casings and drill strings for the purpose of guiding the drilling of a new, or a "relief" borehole to either intersect or to follow a controlled path in the vicinity of a "target" well containing the ferromagnetic material. Robinson et al. U.S. Pat. No. 3,725,777, discloses the use of a "least squares" method, based on magnetostatic theory, to simultaneously fit a curve representing the total measured magnetic field magnitude, including both the earth's field and perturbations, as a function of depth, to a curve representing the difference in readings given by a magnetic compass and a gyrocompass for the direction to "north" also as a function of depth in the well being drilled. Canadian patent No. 1,269,710 to Barnett et al., discloses a method of Fourier analysis of either the axial or the lateral component of localized magnetic poles and dipoles on a well string for distance determination. This latter patent does not, however, address the problem of axially distributed poles; i.e., pole smearing, which is particularly important at close distances when passive magnetic methods of proximity determination are most important. The importance of these effects was evaluated quantitatively by Jones, Hoehn, and Kuckes in SPE paper 14388, SPE Drilling Engineering, 1987.
U.S. Pat. No. 3,745,446 issued to Norris discloses apparatus utilizing the difference in the horizontal field between vertically spaced magnetometers, together with a z-axis measurement to permit the top of a buried casing to be pinpointed. U.S. Pat. No. 4,072,200 issued to Morris et al. discloses a method of analyzing the field perturbations of a magnetic pole which utilizes the difference in the gradient of the axial field component observed at two points in the hole being drilled.
While the foregoing methods have enjoyed some success, in practice each has shown limitations. Furthermore, although active source methods have largely superseded passive methods based on the analysis of static field perturbations, the inherent simplicity of passive methods has appeal and, in addition, has potential importance when short ranges are to be evaluated. In particular, a need often still exists to determine lateral distance and direction between two wells unambiguously when they are in close proximity; i.e., at the critical point just before drilling contact is made between two well bores.